1. Field of the Invention
The present invention generally relates to an optical storage device and an optical device, and more particularly to an optical storage device and an optical device in which a lens oscillates toward an object and away from the object.
2. Description of the Related Art
In the field of optical storage devices, such as a disk apparatus, focus servo control, in which a laser beam is focused on a recording layer of a disk, is performed in order to accurately record data on the disk and to reproduce the recorded data from the disk. The focus servo control system controls the position of an objective lens in order to keep the distance between the objective lens and the disk constant by means of a feedback control loop that uses a focus error signal.
Recently, the focused laser beam spot size is being reduced to raise the recording density. As a result, the distance between the objective lens and the disk is also tending to be reduced.
Further, the detection range of the focus error signal, which can be used as a linear position error along the axis of the objective lens, is within a range of ±1 μm from a just focus in focus point.
Therefore, when a mechanical shock is applied to the disk apparatus and thereby the distance between the objective lens and the disk becomes very short, the focus servo system cannot control the position of the objective lens by means of the focus error signal. As a result, the object lens may collide with the disk.
Therefore, an arrangement is needed in which the objective lens is compulsorily moved away from the disk in order to avoid a collision between the objective lens and the disk when the mechanical shock is detected.
A method which compulsorily moves the objective lens away from the disk is described in, for example, Laid Open Japanese Patent Application Number 8-203108. A disk apparatus described in the Laid Open Japanese Patent Application Number 8-203108 monitors the focus error signal. When a defocusing level greater than a predetermined level is detected, then the total amount of reflected light is monitored. When the total amount of reflected light becomes a level smaller than a predetermined level, it is decided that the objective lens is shifted by a mechanical shock and then the objective lens is compulsorily moved away from the disk.
Examples of detection methods of mechanical shock are a displacement detection method based on the focus error signal as described in the Laid Open Japanese Patent Application Number 8-203108 and an acceleration detection method in which the mechanical shock is detected by means of an acceleration sensor and a focus servo current.
Next, the displacement detection method and the acceleration detection method will be explained.
FIG. 1 and FIG. 2 show a comparison between the displacement detection method and the acceleration detection method. FIG. 2 shows an enlarged part of FIG. 1, which is enlarged along the time axis. FIG. 1(A) and FIG. 2(A) show a relation between a defocus value and elapsed time since the mechanical shock is applied. FIG. 1(B) and FIG. 2(B) show a relation between a control current A and the elapsed time since the mechanical shock is applied FIG. 1(C) and FIG. 2(C) show a relation between the mechanical shock value G and the elapsed time since the mechanical shock is applied. In each figure, the bold line shows an operational waveform of the acceleration detection method and the dotted line shows an operational waveform of the displacement detection method.
For example, as shown in FIG. 1(C) and FIG. 2(C), the mechanical shock is applied at 0 ms, has a half sine wave shape, a maximum value of 100 G and a duration of 2 ms. One G represents the acceleration due to gravity. In a case in which the acceleration detection method is employed, it is possible to start an operation to move the objective lens away from the disk as shown in FIG. 2(B) when the mechanical shock reaches a threshold value of 10 G as shown in FIG. 2(C). However, in a case in which the displacement detection method is employed, the objective lens is compulsorily moved away from the disk when the defocus value becomes, for example, −0.6 μm as shown in FIG. 2(A). Therefore, the start point of moving the objective lens for the displacement detection method is later than that for the acceleration detection method as shown in FIG. 2(B).
As described above, the start point of moving the objective lens for the displacement detection method is later than a point of time when the mechanical shock acceleration is applied. Therefore, the acceleration detection method is used to move the objective lens away from the disk quickly.
Next, a conventional method for moving the object lens away from the disk will be explained.
FIG. 3 shows a block diagram of a focusing mechanism and a focusing control system for the conventional disk apparatus.
A disk 90 is rotated by a spindle motor, which is not shown in FIG. 3. The rotational axis is AA′. A moving head 80 is provided opposite to the disk 90.
FIG. 4 shows a perspective view of the moving head 80 and FIG. 5 shows a sectional view of the moving head 80.
The moving head 80 can be moved in a radial direction of the disk 90 along a line BB′ by a voice coil motor 91.
The moving head 80 has a lens holder 81, a leaf spring 82, a focusing coil 83, an objective lens 84, a rising mirror 85, a moving head body 86 and permanent magnets 87. The objective lens 84 is attached to the lens holder 81. The lens holder 81 is attached to the moving head body 86 by the leaf spring 82 and can oscillate in directions AA′. The focusing coil 83 is attached on a side of the lens holder 81. The permanent magnets 87 are attached to the moving head body 86 opposite to the focusing coils 83.
When the focusing coil 83 is supplied with a driving current, a force is generated in the focusing coil 83 by electromagnetic interaction between the driving current through the coil 83 and a magnetic field caused by the permanent magnet 87. The electromagnetic force generated in the focusing coil 83 can oscillate the lens holder 81 in the directions AA′. As lens holder 81 is oscillated in the directions AA′, the objective lens 84 is also oscillated in the directions AA′. Therefore, it is possible to use the objective lens 84 for focusing control.
The rising mirror 85 is attached to the moving head body 86 under the objective lens 84. The light beam L is supplied to the rising mirror 85 from a stationary head 100. The rising mirror 85 reflects the light beam L supplied from the stationary head 100 to the objective lens 84 in the direction A. The light beam L reflected by the rising mirror 85 is supplied to the objective lens 84. The objective lens 84 focuses the light beam L supplied from the rising mirror 85 on the disk 90.
The light beam L focused on the disk 90 is reflected on the disk 90 and then supplied to the rising mirror 85 through the objective lens 84. The rising mirror 85 reflects the light beam L supplied from the objective lens 84 to the stationary head 100 in the direction B′.
The stationary head 100 has a laser diode 101, a collimator lens 102, a first beam splitter 103, a first servo lens 104, a first photo detector 105, a Foucault prism 108, a second beam splitter 106, a second servo lens 107 and a second photo detector 109.
The laser diode 101 emits a laser light. The laser light emitted by the laser diode 101 is supplied to the moving head 80 through the collimator lens 102 and the first beam splitter 103. The return light beam from the moving head 80 is reflected by the first beam splitter 103 and then is reflected by the second beam splitter 106 and supplied to the Foucault prism 108. The Foucault prism 108 splits the incident light beam. The light beam split by the Foucault prism 108 is focused on the first photo detector 105 by the first servo lens 104. For example, the first photo detector 105 is a quadrant photo diode. A focus error signal generation circuit 51 generates a focus error signal using currents supplied from the quadrant photo diode. Further, the return light beam from the moving head 80 is reflected by the first beam splitter 103 and supplied to the second servo lens 107 through the second beam splitter 106. The second servo lens 107 focuses the incident light beam on the second photo detector 109. For example, the second photo detector 109 is a photo diode divided into two parts. A track error signal generation circuit 41 generates a track error signal using currents supplied from the photo diode divided into two parts.
FIG. 6 shows an output characteristic of a focus error signal when the objective lens is displaced.
The focus error signal has the output characteristic shown with a dotted line in FIG. 6. The horizontal axis shows the displacement from the disk 90. The arrow shows the direction in which the objective lens is moved away from the disk 90. In a part CC′ of the curve between a maximum value and a minimum value shown in FIG. 6, the output voltage varies linearly as the displacement. This part CC′ is used as the focus error signal, that is to say, the error signal of the distance between the position of the objective lens 84 and the focus point.
The focus error signal generated by the focus error signal generation circuit 51 is supplied to a phase compensation circuit 52. The phase compensation circuit 52 filters the focus error signal for stabilizing the servo loop and generates a focus servo signal. The focus servo signal generated by the phase compensation circuit 52 is supplied to an output selection circuit 57. The output selection circuit 57 is supplied with both the focus servo signal from the phase compensation circuit 52 and an objective lens collision prevention signal from an objective lens collision prevention signal generation circuit 55.
The output selection circuit 57 selects the focus servo signal supplied from the phase compensation circuit 52 and supplies the focus servo signal to the focus current driving circuit 50 during a typical focus servo operation. The focus current driving circuit 50 supplies the focus coil 83 with a current based on the focus servo signal. The objective lens 84 is moved by the current so that the focus error signal becomes zero and thereby the focus servo loop is formed.
The output selection circuit 57 selectively supplies the focus current driving circuit 50 with either the focus servo signal supplied from the phase compensation circuit 52 or the objective lens collision prevention signal from the objective lens collision prevention signal generation circuit 55 according to a switching signal supplied from a mechanical shock detection circuit 54. The mechanical shock detection circuit 54 detects an acceleration of the mechanical shock. The mechanical shock detection circuit 54 outputs a high level signal when the acceleration of the mechanical shock is greater than a predetermined value and a low level signal when the acceleration of the mechanical shock is smaller than the predetermined value
The output selection circuit 57 selects the focus servo signal supplied from the phase compensation circuit 52 while the switching signal from the mechanical shock detection circuit 54 has the low level. On the other hand, the output selection circuit 57 selects the objective lens collision prevention signal from the objective lens collision prevention signal generation circuit 55 instead of the focus servo signal when the switching signal from the mechanical shock detection circuit 54 becomes the high level. The focus current driving circuit 50 supplies the focus coil 83 with a constant current which current moves the objective lens away from the disk 90 after the objective lens collision prevention signal from the objective lens collision prevention signal generation circuit 55 is supplied by the output selection circuit 57.
As described above, conventionally, when the mechanical shock detection circuit 54 detects the acceleration of the mechanical shock greater than a predetermined value, then the focus coil 83 is supplied with a constant driving current and the objective lens is moved away from the disk 90.
However, there is an upper limit of a current that can be supplied to the focus coil 83 when the objective lens 84 is moved away from the disk in the conventional disk apparatus. For example, it is assumed that the resistance of the focus coil 83 is 5 Ω. The lens actuator, which is a mechanical part, is damaged if the DC current having a value greater than ±0.2 [A] is maintained in the focus coil 83, because an adhesive which covers the focus coil 83 is burned by Joule heat generated by the DC current. Therefore, in this case described above, the maximum value of the current is ±0.2 [A] while the objective lens 84 is being moved away from the disk.
FIG. 7 shows time response characteristics when a mechanical shock is applied to the disk apparatus. In FIG. 7, the mechanical shock having a half sine shape is applied to the disk apparatus at time zero, said mechanical shock having a maximum acceleration of 50 G (491 m/s2) and a duration of 2 ms, while focus servo control is being performed, and then the mechanical shock is detected, and the collision prevention current of −0.2 [A] is supplied to the focus coil 83. FIG. 7(A) shows the mechanical shock. FIG. 7(B) shows relative displacement between the objective lens and the disk in the direction perpendicular to the disk surface. FIG. 7(C) shows the lens actuator driving current.
In FIG. 7(B), a negative value of the relative displacement shows that the position of the objective lens 84 is nearer to the disk than the position of the focus point and the objective lens is moved toward the disk about 130 μm from the focus point, which is shown by a value of zero, by the mechanical shock. In the case of a conventional objective lens for a wavelength of 650 nm and NA of 0.55, the working distance is about 1.2 mm. Therefore, the objective lens does not collide with the disk until the relative displacement becomes —1200 μm. However, in case of an objective lens for a short wavelength and a large numerical aperture, for example the wavelength of 410 nm and the numerical aperture (NA) of 0.85, the working distance between the objective lens and the disk is less than several tens of μm. Therefore, the objective lens 84 collides with the disk 90 in the case shown in FIG. 5.
In the case that the working distance is 1.2 mm, it is possible to attach a stopper on the moving head body 86 to limit the oscillation of the lens holder 81 in the directions AA′ and thereby to prevent the objective lens from colliding with the disk. However, in the case that the working distance is less than several tens of μm, it is impossible to attach the stopper on the moving head body 86 because the dynamic axial runout of the disk 90 is greater than the working distance.